The main question that Prum seeks to answer in The Evolution of Beauty is what drives the process of sexual selection. Evolution among animals is driven both by what animals manage to survive and who manage to have children and grandchildren. The latter usually translates into males fiercely competing for access to females, both through physical conflict between males and males competing to attract choosy females with ornamentation and courtship rituals. Prum, an ornithologist, is most interested in the latter, which is highly pronounced among numerous species of birds, where only a small percentage of males are able to reproduce. Prum describes many ways that male birds try to lure females, including the elaborate mating dances of the great argus pheasant, the colorful structures of male bowerbirds, and the violin-like sound produced by the club-winged manakin's wings.

The question is why have such elaborate ornamentation and mating rituals evolved. The leading theory is adaptive mate choice, which argues that the features that the female (it's usually the female) selects are honest indicators of good genes. For example, if a female is attracted to males with bright, colorful plumage, this is because such plumage indicates that the male is healthy and will produce healthy offspring.

Prum suggests, rather, that the main reason why the female chooses mates with more more beautiful and elaborate ornamentation or mating rituals is simply because she likes these features. That is, she finds the features beautiful and is attracted to beauty for its own sake. Prum calls this idea Beauty Happens. He supports the idea with a model of a runaway feedback loop proposed by Ronald Fisher, by which small, arbitrary preferences become more pronounced due to females wanting to mate with attractive males and have attractive sons who can also successfully mate. He also supports his theory with numerous examples of mating displays that seem unlikely to provide reliable information about gene quality.

In the last few chapters, Prum turns to applying his principle to the evolution of humans. He speculates on how sexual selection could explain changes such as the reduction in sexual size dimorphism, the rise of paternal parental care, the female orgasm, and homosexuality. This section is quite speculative and Prum's attempts to explain all of these by female mate choice are not always convincing, but they are interesting nonetheless.

The book is also philosophically interesting because his theory that Beauty Happens means that many animals are autonomous agents that drive evolution through their intentional choices. It also hints at the inner life of animals, implying that they take pleasure in perceiving beauty.

One weakness of the book is Prum's misuse of the notion of the null hypothesis. Prum says that Beauty Happens is the null hypothesis, but the null hypothesis is the assumption that any given theory is false. Thus, the null hypothesis is that both his theory, that the aesthetic preferences of animals are a factor in sexual selection, and his opponents' theories are both false

Nonetheless, the book is an interesting read, for the opportunity to see the extremes that evolution can lead to, for Prum's discussion of aesthetic evolution in birds and humans, and for his persuasive defense of aesthetic preference as a factor in evolution.

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